This invention relates to a bulk superconductor such as a superconducting wire useful as electric cable and a superconducting plate useful as a magnetic shield plate and to a process of preparing same.
The recent technology of superconductors can now produce superconductors having Tc of 90K or more, such as Y--Ba--Cu--O and Bi--Sr--Ca--Cu--O, and operable using liquid nitrogen. For the practical applications, however, it is necessary that such superconductors be shaped in the form of wires, tapes, plates, etc. Known such bulk superconductors, however, cannot be put into practice because the superconductive critical current is unsatisfactory.
For instance, a Bi--Sr--Ca--Cu--O tape having a length of more than 100 m is known. This tape cannot be used in practice at a liquid nitrogen temperature. Further, Bi--Sr--Ca--Cu--O crystal has a high structural anisotropy, i.e. the critical current is high when a magnetic field is applied in the direction normal to the c-axis but is low in the direction parallel with the c-axis. Also known is a pancake coil of Bi--Sr--Ca--Cu--O capable of generating a magnetic field of more than 1 T at 20K. At a liquid nitrogen temperature, however, the magnetic field is only 0.1 T.
When a Y--Ba--Cu--O having a Tc higher than the liquid nitrogen temperature is used for forming a plate in accordance with the powder-in-tube method suitably employed in Bi--Sr--Ca--Cu--O superconductors, the crystal grain boundaries form weak links so that superconducting critical current is extremely low.